Extend the techniques of in situ molecular hybridization to the integration of oncornavirus nucleic acid sequences in individual mammalian cells. Iodination of viral nucleic acids to a high specific activity with I-125. Improvement of in situ hybridization techniques. Synthesis of biotinized derivatives of RNA for visualizing RNA-DNA molecular hybrids by autoradiography with the aid of radio-iodinated avidin. Verification of technique by the study of the intrachromosomal integration sites of Rauscher and Gross leukemia viruses. Separation and analysis of RNA sequences from Rauscher and Gross leukemia viruses which hybridize either rapidly or slowly with mouse DNA.